Linked compressor and expander (Lincompex) systems are known in the telecommunications art. A digital Lincompex system is disclosed in U.S. Pat. No. 4,271,499 ('499) issued June 2, 1981, to Leveque, the inventor of the present application and entitled "METHOD AND APPARATUS FOR DIGITALLY IMPLEMENTING A LINKED COMPRESSOR-EXPANDER TELECOMMUNICATIONS SYSTEM". This Leveque '499 patent teaches the transmission of voice over a Lincompex system. Lincompex systems can also be utilized to transmit data (any complex waveform such as multi-tone data). Such a system is disclosed in U.S. Pat. No. 4,907,218 ('218) issued Mar. 6, 1990 also to Leveque, the inventor of the present application, and is entitled "System and Method of Transmitting A Complex Waveform Over A Communication Channel Utilizing Lincompex Techniques".
An exemplary Lincompex system is illustrated in FIGS. 1(A) and (B) of the present application. Information to be transmitted is introduced to an input 20 of the Lincompex system modulator as shown in FIG. 1(A). A control tone generator or envelope circuit 24 monitors the inputted information signal. An envelope detector 26 of the control tone generator (envelope circuit) 24 detects the envelope of the introduced information signal and develops an envelope signal having an voltage representative of the signal level of the introduced information signal. A compressor 22 compresses the inputted information signal. Compression is performed by dividing the signal by its envelope in pseudo-real time to produce a compressed information signal. A control tone is developed by supplying the envelope signal developed from the output of the envelope detector 26 to a logarithmic (log) amplifier 28 which then develops a signal representative of the logarithm of the envelope signal. The output of this log amplifier 28 is supplied to a control terminal of a voltage controlled FM oscillator 30 which generates a frequency that varies about a center frequency F.sub.c in relation to the variation of the input voltage supplied to its control terminal from the log amplifier 28 to develop an envelope signal as an output of the control tone generator (envelope circuit) 24.
A summer 32 then sums the compressed information signal developed at the output of the compressor 22 with the envelope signal developed the output of the voltage controlled FM oscillator 30 to form a combined information signal.
In this speech transmission system, the combined information signal outputted from the summer 32 is provided to a transmitter 34 which transmits the signal over a desired transmission medium 36. In a typical embodiment, a single sideband transmitter would normally transmit the modulated combined information signal through the atmosphere in a known manner.
A Lincompex demodulator is illustrated in FIG. 1(B). The demodulator receives the modulated combined information signal from the transmission medium 36, which normally includes an antenna for receiving radio waves from the atmosphere, supplying the received modulated combined information signal to a receiver 38 which demodulates the transmitted signal to reproduce the combined information signal. Typically, this receiver 38 would be a single sideband receiver which mixes the received modulated combined information signal with the carrier frequency to reproduce baseband combined information signal.
To recover the information from such a combined information signal, a low pass filter 40 removes the envelope information from the combined information signal. This allows the recovery of the compressed information signal containing the information 2. This compressed information is transmitted according to the Lincompex techniques at a substantially constant syllabic peak voltage which enables substantially complete modulation of the transmitter 34 of FIG. 1(A). This information must then be expanded to produce the necessary dynamic range for the recovered information signal to be supplied at the output 60. Accordingly, an expander 42 is utilized which essentially multiplies the compressed voice signal developed at the output of low pass filter 40 by the envelope signal which is recovered by a control tone conversion circuit 44.
The control tone conversion circuit 44 includes a bandpass filter 46 which recovers only the envelope signal from the combined information signal. This frequency modulated envelope signal originally developed by the voltage controlled FM oscillator 30 of FIG. 1(A) is then frequency demodulated by a frequency demodulator or discriminator 48 to recover the logarithm of the envelope. An anti-logarithm amplifier 50 is then utilized to recover the original envelope developed by the envelope detector 26 of FIG. 1(A). This original envelope signal is then used to recover the original voice signal by expanding the compressed voice signal via the expander 42 to provide the original signal to the output 60.
To transmit data having a complex waveform, a system shown in FIGS. 1(A) and 1(B) is modified to include frequency shifting devices which, for example, frequency shift the data bandwidth prior to compression and after expansion. Such a system is readily disclosed in the Leveque '218 Patent. Hereinbelow, a signal to be transmitted comprising either a voice signal or a complex signal such as modulated digital data will be referred to as complex information waveform.
In a Lincompex system such as that described above, the compressor 22 attempts to produce a compressed information signal at its output which is of constant power. However, virtually any introduced information signal provided at the input 20 will have substantial variations in signal amplitude. When the information signal to be transmitted is voice, there will be voice peaks in which the voice to be transmitted is extremely loud and silent passages between syllables and words where the introduced information signal is in essence absent. The compressor 22 attempts to convert this input information signal into a compressed signal having a constant output level. The compressor, however, does not create a signal where there is none and further, since the Lincompex system is a real world system rather than a "ideal" system, the compressor 22 can provide compression over only a limited dynamic range. For example, a preferred embodiment of such a Lincompex system might utilize a compressor which can compress signals within a 48 dB dynamic range. When signals fall outside this dynamic range, complete compression to a constant compressed signal can no longer occur. Consequently, compression is selected to typically occur in a range just below maximum input signal power. Thus, if the maximum input information signal power is at 0 dB, a compression would occur in the range between 0 dB and -48 dB. When the input information signal drops below -48 dB in this example, complete compression no longer occurs and the compressor output varies in proportion to the input information signal level. The performance of a typical compressor 22 is graphically illustrated in FIG. 2. FIG. 2 illustrates the relationship of the input signal to the compressed signal. Such a compressor produces a substantially constant power output signal for input signals within a range below input information signal peak power. Thus, full compression occurs within a range 10 of higher input information signal levels. At a point in time, sometimes known as the compression "knee", the compressor is no longer able to fully compress the input signal to a constant output signal level. Thus, the output of the compressor 22 becomes "linear" with the compress signal varying in proportion to the level of the input signal.
A basic object of Lincompex systems is to improve transmission of transmitted signals by maximizing transmitter frequency to at or near peak transmitter power. This is performed by transmitting the compressed signal near transmitter peak power with a generally constant amplitude control tone signal added thereto. When the input signal drops below the compressor "knee" such that the compress signal power begins to fall, the transmitter no longer transmits at peak power. The compressed signal during these times is reduced in power while the envelope signal remains of constant power.
During quite periods when the compressed signal output is therefore near zero, the total transmitted power of the combined information signal is near that of the control tone which is a relatively small percentage of normal signal power.
At the receiver, during such quite periods, the receiver must lock onto the control tone and will demodulate the content of the frequency band within which the compressed information signal normally resides. When substantially no signal is being transmitted, channel resident noise may approach or even exceed the level of the compressed information signal. If the signal lock is lost on the control tone, this noise can be expanded to a relatively high voltage level during demodulation.
Applicants have discovered that, when the compressed information signal falls below the compression "knee", the performance of a Lincompex system can be substantially enhanced by boosting the power level of the control tone signal. Boosting the power level of the control tone signal enhances the ability of the control tone signal to maintain signal lock. Further, this improvement can be obtained by better utilization of the transmitter power in an area where the compressed information signal has, in any event, lost power when compared to higher input information signal levels. This boosting of the control tone power when the compressed information signal power drops below the compressors maximum output power improves Lincompex performance not only with respect to voice signals but to data signals as well.